An exclusive look at what investigators have discovered about al-Qaeda's plans for its next big attack

The headquarters of Prudential Financial Inc. in Newark, N.J., would not seem like an obvious target for terrorists. It is neither a venerated symbol of American capitalism like the New York Stock Exchange building nor an iconic piece of modern architecture like the Citigroup building in midtown Manhattan, which houses America's biggest bank. It looms over a city that was once the very symbol of urban blight in America. On a clear day the more famous spires of Manhattan are visible from Newark, and so too is the empty space in the skyline on the island's southern tip, where once the...